This invention relates to printing apparatus. More particularly, although not exclusively, it discloses an improved device, which compensates for the stretching of paper substrates during web offset or other printing processes. Although the following description refers to offset printing of paper, the invention need not be limited thereto, as other forms of printing including, for example, textile printing, may benefit from the principles embodied in the invention, by suitable adaptation where necessary.
Web offset printing processes involve the application of pressure and tension forces to the paper substrate as well as the simultaneous use of damping chemicals on the printing plate. The effects of these combine to cause the substrate to stretch laterally following each printing impression. The degree of stretching can vary from a small fraction of a millimetre to as much as two millimetres over a web one metre wide. This constitutes a substantial problem on multi-colour presses where four or more colours are printed in series since the images then fail to accurately superimpose upon one another.
Currently it is known to install small xe2x80x9cbustle wheelsxe2x80x9d under the web path, which protrude upwardly to distort the paper immediately prior to the second and subsequent image prints. The height of the wheels is accurately adjusted to distort the web a measured amount just sufficient to compensate for any stretching. This system has the disadvantage however that the wheels have to actually contact the substrate in order to distort it. The wheels therefore leave a track through the wet ink previously printed and thus they can only be applied to waste areas that are later trimmed away. Also, the distorting force required from the wheels at full production speeds is often so excessive that at idle speed or start up the substrate is ruptured. Accordingly, the position of these wheels has to be monitored and changed with the speed of the substrate.
It is also known to distort the web using jets of gas from fixed nozzles, however this has proved to be difficult to manage as the jets exhaust a massive volume of compressed air and the pressure has to be continuously monitored and adjusted over as many as twenty nozzles with a typical press.
It is an object of the present invention to ameliorate some or all of the aforementioned disadvantages by providing a device for controlling the stretch in a printing web or substrate in which a cushion of air or other suitable gas is provided between the web and the top of a height-adjustable post. At the very least, the invention provides an alternative to presently proposed methods and means of controlling the stretch in printing webs.
According to the present invention, there is provided a printing web distortion device for imposing a measured amount of distortion into a moving web or substrate, such as paper or the like, to compensate for lateral stretching thereof during a printing operation, said device comprising a vertically adjustable post with a gas nozzle disposed at or adjacent to the tip thereof for generating a cushion of gas, a support enclosure for said post with a source for pressurised gas associated therewith, said post being longitudinally slideable within said support enclosure whereby said tip is either raised so that it approaches said substrate in a first operating mode whereby said post imposes said stretch compensating distortion by the application of an elevating force provided by the vertically adjustable post between the tip of which and the moving substrate there is provided the said cushion of gas, especially air, or in a second idle mode, said post is withdrawn from said substrate so that no elevating force is provided on the substrate.
Preferably, the nozzle is formed in the shape of a slot which is transverse to the path of the moving web substrate so as to improve the shape and effect of the air cushion on the substrate.
Preferably, the support enclosure includes a cylinder within which said post is able to slide.
Preferably, said support enclosure includes means for varying the volume and/or pressure of the gas emanating from said nozzle in accordance with the position of the post within said enclosure, that is to say the relative height to which the post is adjusted when in its operating mode. More preferably, said means for varying the volume and/or pressure of said gas includes a series of ports that are progressively uncovered as the post approaches the substrate. In this way, a suitable void is maintained between said post and said substrate, irrespective of the height of the post, since increasing resistance is encountered from said substrate with increasing height of the post (ie with increased distortion of the substrate), which would otherwise tend to cause the substrate to be drawn down onto the tip of the post. Variation in gas flow is thereby achieved as a result of the operation of the device itself and no adjustment is required to the external air pressure supply.
Thus, in a preferred embodiment, the post includes an internal central conduit, one end of which is in communication with said nozzle and the other end, during use of said device, is in communication with said source of pressurised gas and wherein said means for varying the volume and/or pressure of said gas comprises a plurality of spaced ports, preferably of varying diameters, disposed in the path of the gas, one or more of which are progressively uncovered as the height of the post is increased. During use of said device displacement of said post operates to vary the number of or choice of port or ports in communication with said source of pressurised gas and thus to increase the volume and/or pressure of gas emanating from said nozzle as the post rises.
Preferably, in one embodiment of the invention, there is provided a post assembly comprising a post, a downwardly depending stem integral therewith and adjustment means associated therewith to adjust the height to which the post may be extended. The means to raise the post to the operating position, is provided by means of gas pressure acting on the base of the post. This is achieved, for example, by providing a chamber which is formed between the base of said post and a sealing collar which is suitably secured to the lower extremity of the said support enclosure and into which compressed air is introduced. A retaining means is fastened to the lower extremity of said support enclosure to secure the sealing collar. Preferably the post is returned to its idle position by means of a coil spring operating on the post assembly when the gas pressure is turned off and the pressure exhausted. It will be appreciated that the sealing collar limits the downward travel of the post assembly.
Preferably, the means to limit the height to which the post extends during the operating mode (ie when operated upon by the gas pressure), is provided by adjustment means in the form of a screw adjustment facility operating directly on the post. This is achieved in the present embodiment wherein the post is provided with the downwardly depending stem which passes through the sealing collar, and which is linked by means of a screw thread on the lower end of said stem which mates with a corresponding thread located internally in an adjustment knob which is able to be rotated progressively to vary the amount by which the tip of the said post is able to extend above the support enclosure, thus determining the height to which the post may be raised under the influence of the gas pressure.
The above mentioned retaining means furthermore provides a mechanical stop for the upward travel of the rotatable adjustment knob, thereby limiting the upward travel of the post assembly, the extent to which the post itself is able to be raised being determined by the relative position of the adjustment knob. The coil spring is conveniently located between a lower surface of the retaining means and an upper surface of the adjustment knob. It will be readily appreciated that several mechanical equivalents of such an arrangement are possible, including the provision of appropriate recesses within either or both of the retaining means or adjustment knob to accommodate the spring. Indeed, where the adjustment knob itself is partly located within a recess in the retaining means, a graduation scale may be incorporated to facilitate accurate adjustment.
In this embodiment, the components interact to form a device, in which the post has a continuously adjustable travel stroke within a desired range, and which is operable between a predetermined adjustable height to cause the required distortion of the substrate during printing. When deactivated the post will reliably return to its idle or home position.
It will be further understood that whilst it is generally preferable to provide independent supplies of compressed gas to lift the post on the one hand and to provide the cushion of gas at the tip of the post on the other, it will also be possible to have a single supply carry out both functions by having the central conduit described above extend downwardly into the aforementioned chamber. There are however limitations arising from this arrangement due to the need to more accurately control the rate of flow of compressed gas.
In an alternate embodiment, the means to raise the post, lower the post to its idle position and provide adjustment in the height to which it is raised are accomplished by means of a motor instead of gas pressure, return coil spring and adjustment knob.
Thus in one preferred form of this alternate embodiment, the post itself is provided with an internally threaded area which is engaged by means of its screw threads to a lead screw fitted to a motor which is secured to the base of said support enclosure whereby rotation of the motor and thus the fitted lead screw displaces the post on said threads, causing it to travel up or down within the support enclosure in order to withdraw the post from or cause it to approach said substrate. It will be appreciated that both the aforementioned adjustment in the height to which the post may be raised, as well as the mechanism by which the post is raised and returned between its idle and operating positions is thereby accomplished through the use of the motor as a positive drive.